Can You Use Spar Urethane Sealer On Outdoor Pine Wood?

Pine is a popular wood for framing thanks to it’s surprising stability. There’s very little that can cause this softwood to warp and twist — not even changes in humidity.

Yet, despite this, Pine wood is not a rot-resistant timber. And if it’s exposed to the rainy/heat cycles of the great outdoors, it can start to decay.

So, sealing exterior Pine wood is pretty much a necessity, if you want it to last. But, is Spar Urethane really the right sealant for the job?

Well, in this post you will learn how Spar Urethane can make Pine wood more durable and weatherproof. You will also find out if Spar Urethane can work as a knot sealer.

And keep reading to discover why you should avoid using a foam brush to apply this exterior wood sealant.

spar urethane on pine

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What’s The Best Finish Or Sealer I Can Use To Protect Outdoor Pine Wood?

Spar Urethane easily pulls out ahead as the winner (when it comes to protecting exterior wood). And it is so water-resistant that its regularly used to seal boat decking and marine-side structures.

It’s also very durable. So much so, that it only needs reapplying once or twice a decade. Compare that to say Tung oil or Boiled Linseed oil, both which need reapplying every 6-12 months.

On top of that, Spar Urethane sealants often come with added weatherproofing ingredients. And those ingredients can help prevent Pine wood from wood rot and UV-damage.

Does Spar Urethane Darken Wood At All? If you are using a clear water-based Spar Urethane, then no it won’t. Water-based Spar Urethane doesn’t yellow wood the way that an oil-based Spar Urethane might. And those added UV-blockers in this exterior sealant can prevent UV-damage discoloration too.

OK. But Does Spar Urethane Make Pine Wood Tougher And More Durable?

In a round about way, it does.

Pine wood is a very stable timber, meaning it does not warp and twist due to changes in humidity. However, this softwood is not rot-resistant (unlike hardwoods such as Teak or Rosewood).

So, if you plan on using Pine wood for an outdoor structure, it will need all the help it can get. Which is why Spar Urethane can make Pine wood more durable.

But, it doesn’t make Pine wood tougher, since Spar Urethane does not reinforce timber the way a wood hardener can. Instead, Spar Urethane wraps the surface of wood with a scratch-resistant coat that helps to keep out moisture.

But, that coat has more flexibility to it than other urethane-based sealers (such as polyurethane). And that added flexibility makes Spar Urethane even better at shrugging off scrapes and scratches.

Related Post: Can You Apply Spar Urethane Over Paint? (Or Should You Leave Painted Wood Unsealed)

And How Do You Apply It? Should You Apply Spar Urethane Using A Foam Brush?

If you want a bubble-free surface, stick to using a bristle brush. Foam brushes can add lots of troublesome bubbles to a drying Spar Urethane sealer, if you aren’t careful.

And, if you’re worried about air bubbles appearing in your Spar Urethane finish, you should check out our post right here on The Woodwork Place: What’s With All These Bubbles In My Spar Urethane Finish?! [What You Need To Know]

Can Spar Urethane Seal Those Pine Wood Knots As Well?

Any water-resistant sealant, including Spar Urethane, will seal over wood knots. Which is a good thing, because knots can ‘bleed’ out tree sap, and discolor wood stains or coats of paint.

But, just because it can seal knots, doesn’t mean that Spar Urethane is the best product for the job.

Ideally, you should use a wood knot blocker to seal Pine wood knots instead. A quality wood knot blocker will act like a primer, sealing the knots and prepping the surface for a top coat finish.

How Do You Seal Pine Wood Without Yellowing? Coat it with a clear water-based Spar Urethane product. Oil-based spar urethanes tend to yellow as they age.

To Wrap Up, Here Are The 3 Key Takeaways From This Post…

  • 1). Pine wood is stable, but is not rot-resistant. It will need a water-resistant wood sealer, if it’s to last outside.
  • 2). Spar Urethane is a water-resistant wood sealant. It is widely used to seal marine-side structures and boat decking.
  • 3). Spar Urethane can be used to seal wood knots. But, you’re better off using a wood knot blocker to seal Pine wood knots.


Yang, Han-Seung, et al. “Water absorption behavior and mechanical properties of lignocellulosic filler–polyolefin bio-composites.” Composite structures 72.4 (2006): 429-437.